High School

Melvin is counting how much medication he has left in his pill vial and realizes that he has only 6 tablets left. His prescription says to take a 5 mg tablet orally every 8 hours for 3 weeks. He has been taking the medication for only 12 days now. How many tablets could he be missing if he was given the exact number of tablets for the course of treatment?

Explain how you arrived at your final answer.